The present invention relates to a safety apparatus for a nuclear reactor, particularly a fast neutron reactor whose core, cooled by the circulation of a liquid metal, in general sodium, has a central area called the fuel area and a peripheral covering area surrounding the said central area.
The control of the safety of a nuclear reactor of this type and in particular the immediate stoppage of the nuclear reaction in the case of incidents or accidents of various types which may possibly occur during operation necessitates the use of reliable effective means which in conventional manner comprise neutron-absorbing elements which can be introduced into the core and in particular into the fuel area within a very short time, whereby a sufficient number of said absorbing elements distributed throughout the core are provided to very rapidly stop the reaction.
In the U.S. patent application Ser. No. 521,900 filed on Nov. 7, 1974. already describes an apparatus of this type which comprises an absorbing system equipped with a support head connected by a disconnectable linkage to the lower end of an axially movable vertical operating rod, whereby after the disconnection of the disconnectable linkage said absorbing system is able to slide by gravity from an upper position into a lower position in a passage which is defined through the reactor core by a container which is open at its upper end and is arranged. in the core among other containers containing the nuclear fuel. In this apparatus the disconnectable linkage between the head of the absorbing element and the end of the operating rod is arranged, with the element in the upper position, in the upper part and at least partly within the open container, the absorbing element extending into said container in the core covering area outside the central area.
Preferably the disconnectable linkage is realised by means of an electromagnet armature fitted at the lower end of the operating rod. The absorbing element is constructed in articulated form by means of absorbing links joined to one another beneath the head of the element and cooperating in the lower engagement position in the core with an internal abutment of the container, whereby a shock absorber system is preferably provided at the end of the travel.
However, such an apparatus which is particularly reliable with respect to its principle of operation may still be rendered defective. Thus, one of the possible incidents requiring the immediate dropping of the absorbing element may simultaneously consist of the stoppage of the cooling of the core due to a blockage of the liquid metal circulating pumps, and in the absence of a command from the electromagnet the absorbing element will be released. Although the possibility of these two faults occurring on the apparatus at the same time is almost non-existent, it is still necessary to provide a safety apparatus which ensures a completely automatic dropping of the absorbing element and which in all cases comes into operation as soon as there is an abnormal rise in the temperature of the liquid metal.